Don't have experience with this myself, but:
5. It is much easier to sell a share rather than a whole aircraft when you decide to leave.
If you try to sell a single share in an established group, then the potential group of people that might want to buy your share is limited to, say, a 100km radius around the airfield where it's based. Whereas if you try to sell a full aircraft, particularly if it's properly maintained and on an ICAO CofA or something, the whole world is a potential buyer.
Mind you, some groups have provisions for buying back your share at a certain (reduced) price in certain circumstances.